Expandable styrene polymers containing graphite particles

ABSTRACT

Particulate expandable styrene polymers contain homogeneously distributed graphite particles and can be processed to give self-extinguishing foams having a density of &lt;=35 g/l.

The present invention relates to particulate, expandable styrenepolymers containing graphite particles, their production and foamsproduced therefrom.

Expanded polystyrene foams have been known for a long time and haveproven themselves in many fields. Such foams are produced by foamingpolystyrene particles impregnated with blowing agents and subsequentlywelding together the foam particles produced in this way to givemoldings. A significant application area is thermal insulation inbuilding and construction.

In many applications of foams, in particular in building andconstruction, it is a requirement that the foams be self-extinguishing.Although it is known that this can be achieved by addition of flameretardants, e.g. bromine compounds, whether a foam passes a particularburning test depends on various factors such as composition and densityof the foam, type and amount of flame retardant and also type and amountof further additives.

The foam boards made of expanded polystyrene foam which are used forthermal insulation usually have densities of at least 30 g/l, since thethermal conductivity of the expanded polystyrene foam has a minimum atthese densities. To save material, it would be desirable to use foamboards having lower densities, in particular ≦15 g/l, for thermalinsulation. The production of such foams is not a problem in technicalterms. However, such foam boards having a lower density have adrastically worse thermal insulation performance, so that they do notmeet the requirements for thermal conductivity class 035 (DIN 18 164,Part 1).

It is known that the thermal conductivity of foams can be reduced byincorporation of athermanous materials such as carbon black, metaloxides, metal powder or pigments.

Thus, EP-A 372 343 describes polystyrene foams containing from 1 to 25%by weight of carbon black. The carbon black has a particle size of from10 to 100 nm and a surface area of from 10 to 1500 m²/g. The polystyrenefoams described there are predominantly produced by the extrusion methodand preferably have a density of 32-40 g/l, as is typical for thesefoams. The addition of flame retardants is mentioned, but the expandedpolystyrene foams described in the examples containing 1.7% by weight ofhexabromocyclododecane do not pass the burning test B2 in accordancewith DIN 4102.

WO 94/13721 describes similar foams in which the size of the carbonblack particles is >150 nm.

EP-A 620 246 describes moldings made of expanded polystyrene foam whichcontain a particulate athermanous material, in particular carbon black,and also graphite. The density of the moldings is less than 20 g/l. Theparticles are preferably incorporated into the moldings by surfacecoating of the prefoamed polystyrene beads or by embedding into the notyet foamed polystyrene granules. However, this distribution of theparticles on the surface leads to severe impairment of the weldingtogether of the prefoamed beads and consequently to foams of lowquality; in addition, the particles can be rubbed off from the surfaceof the molding. In both cases, the particles are in any case nothomogeneously distributed in the interior of the polystyrene particles;an addition of flame retardants is not described.

A similar process is described in GB-A 1 588 314, according to whichantistatic polystyrene foams are produced by coating unfoamed orprefoamed particles with a graphite suspension.

It is an object of the present invention to provide expandable styrenepolymers containing graphite particles which can be processed to giveexpanded polystyrene foams which have both a low density and a lowthermal conductivity and have good processing properties, good physicalproperties and, in particular, very good flame retardant properties.

We have found that this object is achieved by particulate, expandablestyrene polymers containing homogeneously distributed graphite particlesand can be processed to give foams which have a density of <35 g/l andare preferably self-extinguishing and pass the burning test B2 (inaccordance with DIN 4102).

The invention also provides processes for producing the expandablestyrene polymers and also provides the expanded polystyrene foamsproduced therefrom.

For the purposes of the present invention, expandable styrene polymersare styrene polymers containing blowing agents.

The polymer matrix present in the expandable styrene polymers of thepresent invention is, in particular, homopolystyrene or a styrenecopolymer containing up to 20%, based on the weight of the polymers, ofethylenically unsaturated comonomers, in particular alkylstyrene,divinylbenzene, acrylonitrile or α-methylstyrene. Blends of polystyreneand other polymers, in particular with rubber and polyphenylene etherare also possible.

The styrene polymers can contain the customary and known auxiliaries andadditives, for example flame retardants, nucleating agents, UVstabilizers, chain transferrers, blowing agents, plasticizers, pigmentsand antioxidants.

The expandable particles are coated with the customary and known coatingmaterials, for example metal stearates, glyceryl esters and finelydivided silicates.

The particle size is preferably in the range 0.2-2 mm.

The graphite used preferably has a mean particle size of from 1 to 50μm, in particular from 2.5 to 12 μm, a bulk density of from 100 to 500g/l and a specific surface area of from 5 to 20 m²/g. Either naturalgraphite or milled synthetic graphite can be used. The graphiteparticles are preferably present in the styrene polymer in amounts offrom 0.05 to 25% by weight, in particular from 2 to 8% by weight.Surprisingly, it has been found that graphite particles are effectiveeven in amounts of less than 0.5% by weight.

A problem associated with the use of graphite particles is the readyflammability of the expanded polystyrene foams containing graphiteparticles. Thus, graphite-containing polystyrene foams have hitherto notbeen able to pass the burning tests required for use in building andconstruction (B1 and B2 in accordance with DIN 4102).

To rectify this defect, flame retardants, particularly ones based onorganic bromine compounds, are added to the expandable styrene polymersin a preferred embodiment of the invention. The bromine compound(without a synergist) should be added in an amount of more than 3% byweight, based on the weight of the expandable styrene polymers. B1 andB2 are not passed when the customary amount of flame retardant is used.The organic bromine compounds should have a bromine content of ≧70% byweight.

Surprisingly, this amount of flame retardants leads to no deteriorationwhatever in the mechanical properties of the expanded polystyrene foamscontaining carbon black.

Particularly suitable flame retardants are aliphatic, cycloaliphatic andaromatic bromine compounds, for example hexabromocyclododecane,pentabromomonochlorocyclohexane and pentabromophenyl allyl ether.

The effect of the bromine-containing flame retardants is considerablyimproved by addition of C—C- or O—O-labile organic compounds. Examplesof suitable flame retardant synergists are bicumyl and dicumyl peroxide.A preferred combination comprises 0.6 to 5% by weight of an organicbromine compound and 0.1 to 1.0% by weight of the C—C- or O—O-labileorganic compound.

The expandable styrene polymers of the present invention can be producedby various methods.

In a preferred embodiment, the graphite particles are mixed with a meltof the styrene polymer, preferably in an extruder. At the same time, theblowing agent is metered into the melt. The graphite particles can alsobe compounded into a melt of styrene polymer containing blowing agent;in this case, it is convenient to use oversize and undersize fractionsof polystyrene beads containing blowing agent formed in a suspensionpolymerization. The polystyrene melt containing blowing agents andgraphite particles is extruded and granulated to form granulescontaining blowing agent. Since graphite has a strong nucleating action,the compounded polystyrene should be quickly cooled under pressure afterextrusion in order to avoid foaming. For this reason, an underwatergranulation under pressure is advantageously carried out.

It is also possible to add the blowing agent to styrene polymerscontaining graphite particles in a separate process step. Here, thegranules are then impregnated with the blowing agent, preferably inaqueous suspension.

In all three cases, the finely divided graphite particles can be addeddirectly to the polystyrene melt. The graphite particles can also beadded in the form of a concentrate in polystyrene. However, preferenceis given to introducing polystyrene granules and graphite particlestogether into an extruder, melting the polystyrene and mixing it withthe graphite.

It is in principle also possible to incorporate the graphite particlesduring the course of the suspension polymerization. Here, they can beadded prior to suspending the monomeric styrene or added to the reactionmixture during the course of the polymerization, preferably during thefirst half of the polymerization cycle. The blowing agent is preferablyadded during the course of the polymerization, but it can also beincorporated into the styrene polymer afterwards. Here, it has beenfound that it is favorable in terms of the stability of the suspensionfor a solution of polystyrene (or an appropriate styrene copolymer) instyrene (or the mixture of styrene with comonomers) to be present at thebeginning of the suspension polymerization. Preference is given tostarting from a 0.5-30% strength by weight, in particular from 5 to 20%strength by weight, solution of polystyrene in styrene. This can beachieved by dissolving fresh polystyrene in monomers, but use isadvantageously made of oversize and undersize fractions which have beensieved out in the fractionation of the variously sized beads obtained inthe preparation of expandable polystyrene. In practice, such otherwiseunusable oversize and undersize fractions have diameters of greater than2.0 mm or less than 0.2 mm. Recycled polystyrene and recycledpolystyrene foam can also be used. Another possibility is toprepolymerize styrene in bulk up to a conversion of from 0.5 to 70% andto suspend the prepolymer together with the graphite particles in theaqueous phase and complete the polymerization.

The blowing agent is added in the customary amounts of about 3-10% byweight, based on the weight of the polymer. As blowing agents, use isusually made of aliphatic hydrocarbons having from 3 to 10, preferablyfrom 4 to 6, carbon atoms.

The novel expandable styrene polymers containing carbon black can beprocessed to produce polystyrene foams having densities of 5-35 g/l,preferably from 8 to 25 g/l and in particular 10-15 g/l.

For this purpose, the expandable particles are prefoamed. This isusually achieved by heating the particles by means of steam inprefoamers.

The particles which have been prefoamed in this way are then weldedtogether to give moldings. For this purpose, the prefoamed particles areintroduced into molds which do not close in a gastight manner and aretreated with steam. After cooling, the moldings can be taken from themold.

A further surprising effect of the addition of graphite particles isthat it can reduce the cooling time until welded foam blocks can beremoved from the mold. Thus, for example, an addition of from 0.5 to 5%by weight of graphite leads to a shortening of from 10 to 90% in thecooling time.

The foams produced from the expandable styrene polymers of the presentinvention have an excellent thermal insulation capacity. This effect isparticularly distinct at low densities. Thus, addition of 2% by weightof graphite to an expandable styrene polymer enables the thermalconductivity at a foam density of 10 g/l to be reduced from 44 mW/m·K toless than 35 mW/m·K.

The present invention further provides expanded polystyrene foams whichhave a density of <35 g/l and contain from 0.05 to 25% by weight ofhomogeneously distributed graphite particles and whose thermalconductivity is reduced sufficiently for the foams to meet therequirements of thermal conductivity class 035 (in accordance with DIN18 164, Part 1, Table 4) and are preferably self-extinguishing and passthe burning test B2 (in accordance with DIN 4102).

The ability to reduce the density of the styrene polymers significantlyat the same thermal conductivity allows material savings to be realized.Since, compared to conventional expandable styrene polymers, the samethermal insulation performance can be achieved at significantly lowerdensities, the expandable polystyrene particles produced according tothe present invention makes it possible to use thinner foam boards,which makes a space saving possible.

Surprisingly, the expandable styrene polymers of the present inventioncan be processed without any problems at all to give low-density foams.There are neither blowing agent losses nor disruptions of the cellstructure of the foams, although a person skilled in the art would haveto assume that the graphite would act as nucleating agent and lead toundesirably fine cells in the foam and poor welding together. Inaddition, self-extinguishing foams which pass the burning test B2 and inmost cases also B1 can be produced despite the addition of graphiteparticles.

Owing to the incorporation of the graphite particles in the polymermatrix, there is no rubbing-off of the graphite and thus no soiling whenworking with such components.

The foams of the present invention can be used for thermal insulation ofbuildings and parts of buildings, for thermal insulation of machines anddomestic appliances and also as packaging materials.

The invention is illustrated in more detail by the examples below. Partsand percentages are by weight.

EXAMPLE 1

In a pressure-resistant stirred vessel, 0.498 kg of oversize/undersizeEPS is dissolved in 16.6 kg of styrene. 16.6 g of pulverulent graphite(Graphitwerk Kropfmühl KG, UF2 96/97), i.e. 0.1% of graphite based onthe total amount of styrene and EPS, are homogeneously suspended in thesolution and 83.0 g of dicumyl peroxide, 4.15 g of dibenzoyl peroxideand 112.033 g of hexabromocyclododecane (HBCD) are added. The organicphase is introduced into 19.3 l of deionized water in a 50 l stirredvessel. The aqueous phase comprises 46.127 g of sodium pyrophosphate and86.348 g of magnesium sulfate (Epsom salts). The suspension is heated to80° C. over a period of 140 minutes. 2.32 g of emulsifier K 30/40 (BayerAG) are then added. After a further 40 minutes, 1330 g of pentane aremetered in and the polymerization is completed at 126° C.

Separating off the aqueous phase gives homogeneously grayish beadshaving a mean diameter of 1.18 mm. Prefoaming the beads twice usingsteam results in a bead density of 10.0 g/l. The internal water contentis <1.5% and the residual styrene content is <1000 ppm. The prefoamedbeads are welded together by means of steam to give foam blocks. Thethermal conductivity at a density of 10 g/l (Poensgen method) is 42mW/m·K.

EXAMPLE 2 COMPARISON

Example 1 was repeated without addition of graphite. The thermalconductivity at a density of 10 g/l was 44 mW/m·K.

EXAMPLE 3

In a pressure-resistant stirred vessel, a mixture of 150 parts ofdeionized water, 0.1 part of sodium pyrophosphate, 100 parts of styrene,0.45 part of benzoyl peroxide, 0.15 part of tert-butyl perbenzoate and 5parts of Kropfmühl graphite powder UFZ 99.5, 2 parts ofhexabromocyclododecane (HBCD) and 0.4 part of dicumyl peroxide washeated to 90° C. while stirring.

After 2 hours at 90° C., 4 parts of a 10% strength aqueous solution ofpolyvinylpyrrolidone were added.

The mixture was then stirred for another 2 hours at 90° C. and 7 partsof a mixture of 80% of n-pentane and 20% of iso-pentane were added. Themixture was subsequently stirred for 2 hours at 110° C. and finally for2 hours at 140° C.

The expandable polystyrene beads obtained were washed with deionizedwater, sieved to 0.7-1.0 mm and subsequently dried using warm air.

The beads were prefoamed by treatment with flowing steam and, afterstorage for one day, were welded together in a closed mold by means offurther treatment with steam to give foam blocks having a density of 15g/l.

The measurement of the thermal conductivities was carried out at 10° C.in accordance with DIN 52612. A value of 34 mW/m·K was obtained.

EXAMPLE 4

2.55 kg of polystyrene (PS 158 K from BASF) are dissolved in 17.03 kg ofstyrene. 196 g of pulverulent graphite (Graphitwerk Kropfmühle KG, UF296/97), i.e. 6% of graphite based on the total amount of styrene andpolystyrene, are homogeneously suspended in the solution and 59.6 g ofdicumyl peroxide and 20.4 g of dibenzoyl peroxide are added. The organicphase is introduced into 19.5 l of deionized water in a 50 l stirredvessel. The aqueous phase comprises 69.8 g of sodium pyrophosphate and129.5 g of magnesium sulfate. 195.8 g of pentane are metered into thesuspension which is then heated to 80° C. After 140 minutes, 3.51 g ofemulsifier K 30/40 (Bayer AG) are added. After a further 30 minutes,another 1175.1 g of pentane are metered in and polymerization iscompleted at 134° C. Separating off the aqueous phase giveshomogeneously dark beads having a mean diameter of 0.82 mm. The beadscan be foamed using steam to give a density of 10.2 g/l after 3 minutes.The measurement of the thermal conductivity was carried out on foamblocks at 10° C. in accordance with DIN 52 612. The result is shown inTable 1.

EXAMPLE 5

Example 4 was repeated using 4% of graphite.

EXAMPLE 6

Example 4 was repeated using 2% of graphite.

EXAMPLE 7

Example 4 was repeated using 1% of graphite.

EXAMPLE 8

Example 4 was repeated using 0.5% of graphite.

EXAMPLE 9

Example 4 was repeated using 0.2% of graphite.

EXAMPLE 10 Comparison

Example 4 was carried out without addition of graphite.

TABLE 1 Thermal conduc- Graphite Density tivity Example % g/l mW/m · K 46 10.2 32 5 4 10.0 33 6 2 10.5 35 7 1 10.8 36 8 0.5 10.2 38 9 0.2 10.040 10  — 10.2 44

EXAMPLE 11

Example 4 was repeated with 127 g of hexabromocyclododecane and 85 g ofbicumyl being added as flame retardant system. The polymerization wascarried out at 125° C. A thermal conductivity of 34 mW/m·K was obtained.The fire protection class B2 was achieved.

EXAMPLES 12 to 14

Polystyrene having a mean molecular weight (M_(w)) of 220,000 (PS 148 HBASF) and containing 2.1% of HBCD and 0.42% of bicumyl was plasticizedat 180° C. with addition of the amount of graphite indicated in Table 1as a 20% strength masterbatch in polystyrene in a heated twin-screwextruder and extruded through a die plate having 1 mm diameter orifices.The extrudates were solidified in a water bath and subsequentlygranulated to a particle size of 2×2×2 mm by means of rotating knives.

6000 g of these granules together with 21,300 g of deionized water, 76 gof sodium pyrophosphate, 155 g of magnesium sulfate heptahydrate and 50g of a 40% strength solution of an alkylbenzenesulfonate (Mersolat K 30,Bayer AG) were placed in a 50 l capacity stirred vessel.

The vessel was closed and heated to 120° C. while stirring at 250 rpm.After this temperature had been reached, 500 g of a mixture of 80% ofn-pentane and 20% of iso-pentane were injected into the vessel over aperiod of 15 minutes and the mixture was stirred for another 6 hours at120° C. The expandable beads obtained were washed, sieved to 0.7-1 mm,dried and processed to form foam blocks. At a density of 10.1 g/l, athermal conductivity of 32 mW/m·K was obtained. The B2 test was passed.

In Example 13, the flame retardant was left out. The B2 test was notpassed.

In Comparative Example 14, the graphite was left out. The thermalconductivity was 43 mW/m·K.

EXAMPLES 15 to 18

In a heated twin-screw extruder, 2% of graphite and 5.0% of a mixture of80% of n-pentane and 20% of iso-pentane were metered into moltenpolystyrene having a mean molecular weight (M_(w)) of 220,000 andcontaining 2.1% of HBCD and 0.42% of bicumyl at a melt temperature ofabout 160° C. The homogenized mixture was, at a melt temperature of 180°C., extruded through a die plate having 0.8 mm diameter orifices. Anunderwater granulator was fitted to the die plate. The melt coming outof the die was granulated at a pressure of 5 bar by means of a rotatingknife. Beads having a diameter of 1.5 mm were obtained.

In Example 15, foaming was carried out to a density of 10.3 g/l and inExample 16 (shorter steam treatment time) to a density of 15 g/l. Thethermal conductivities were 34 and 32 mW/m·K, respectively. The B2 testwas passed in each case.

In Example 17, the flame retardant was left out and in ComparativeExample 18 both the flame retardant and the graphite were left out.

The thermal conductivities were 34 and 44 mW/m·K, respectively. The B2test was not passed in either case.

EXAMPLE 19

Polystyrene PS 158 K (BASF AG) was metered into a twin-screw extruder(ZSK 53) together with 2% of graphite, 1.4% of HBCD and 0.7% of bicumyl.In addition, 5% of pentane was mixed into the melt in the extruder. Themelt leaving the extruder die was granulated by means of an underwatergranulator from Gala (USA). Granulation was carried out under a pressureof 5 bar. This pressure was achieved by means of a throttle (hose havinga length of 50 m) installed between granulation and dryer. This gavebead-like black granules having a mean diameter of about 1.5 mm.Moldings produced by foaming and sintering of the foam particles had adensity of 13 g/l and a thermal conductivity of mW/m·K.

COMPARATIVE EXAMPLE 20 Surface Coating with Graphite

In a mixing apparatus, prefoamed EPS beads were admixed with 2.0% ofgraphite. Incomplete coating and nonuniform distribution of the graphiteon the bead surface were observed. During further processing,considerable rubbing-off of the graphite from the bead surface occurred.Use of binders (glyceryl stearate, white oil) was not able to achieveany qualitative improvement in the coating results. The welding togetherto form moldings was unsatisfactory.

What is claimed is:
 1. A particulate expandable styrene polymer whichcan be processed to give foams having a density of ≦35 g/l and containsfrom 0.05 to 8% by weight of homogeneously distributed graphiteparticles having a mean particle size of from 1 to 50 μm.
 2. Aparticulate expandable styrene polymer as claimed in claim 1 whichcontains an organic bromine compound having a bromine content of ≧70% byweight as flame retardant and can be processed to giveself-extinguishing foams which pass the burning test B2 (in accordancewith DIN 4102).
 3. A particulate expandable styrene polymer as claimedin claim 1 which contains from 0.6 to 5% by weight, based on thepolymer, of an organic bromine compound having a bromine content of ≧70%by weight as flame retardant and from 0.1 to 1.0% by weight, based onthe polymer, of a C—C- or O—O-labile organic compound as flame retardantsynergist.
 4. A particulate expandable styrene polymer as claimed inclaim 3, wherein the organic bromine compound is selected from the groupconsisting of brominated aliphatic, cycloaliphatic and aromaticcompounds.
 5. A particulate expandable styrene polymer as claimed inclaim 3, wherein the flame retardant synergist is bicumyl or dicumylperoxide.
 6. A particulate expandable styrene polymer as claimed inclaim 1 which contains from 3 to 10% by weight of a hydrocarbon havingfrom 3 to 10 carbon atoms as blowing agent.
 7. A process for producingan expandable styrene polymer as claimed in claim 1, which comprisesmixing graphite particles and blowing agent with molten polystyrene inan extruder and then extruding the melt and cooling and granulating it.8. A process for producing an expandable styrene polymer as claimed inclaim 1, which comprises mixing graphite particles with moltenpolystyrene containing blowing agent in an extruder and then extrudingthe melt and cooling and granulating it.
 9. A process for producing anexpandable styrene polymer as claimed in claim 1, which comprises mixinggraphite particles with molten polystyrene in an extruder, thenextruding the melt and cooling and granulating it and subsequentlyimpregnating the granules in aqueous suspension with blowing agents. 10.A process for producing polystyrene foams, which comprises foamingexpandable styrene polymers as claimed in claim 1 containing graphiteparticles to a density of ≦35 g/l.
 11. An expanded polystyrene foamhaving a density of ≦35 g/l, containing from 0.05 to 8% by weight ofhomogeneously distributed graphite particles having a mean particle sizeof from 1 to 50 μm and having a thermal conductivity which is reducedsufficiently for the foam to meet the requirements of thermalconductivity class 035 (in accordance with DIN 18164, Part 1, Table 4).12. An expanded polystyrene foam as claimed in claim 11 having a densityof ≦35 g/l and containing, in each case based on the polymer, from 2 to8% by weight of graphite particles, from 0.6 to 5% by weight of anorganic bromine compound having a bromine content of ≧70% by weight, andfrom 0.1 to 1.0% by weight of a C—C— or O-O—labile compound,

which is self-extinguishing and meets the burning test B2 (in accordancewith DIN 4102) and whose thermal conductivity is reduced sufficientlyfor it to meet the requirements of the thermal conductivity class 035(in accordance with DIN 18 164, Part 1, Table 4).
 13. A particulateexpandable styrene polymer as claimed in claim 1, wherein the graphiteparticles have a size of 2.5 to 12 μm.
 14. A particulate expandablestyrene polymer as claimed in claim 4, wherein the organic brominecompound is hexabromocyclododecane, pentabromomonochlorocyclohexane orpentabromophenyl allyl ether.
 15. A method of providing buildings withthermal insulation, comprising: insulating said buildings with theexpanded styrene polymer of claim
 11. 16. A method of constructingbuildings, comprising: in the construction of said buildings, employingstructural materials fabricated from the expanded styrene polymer ofclaim
 11. 17. A method of providing machines with thermal insulation,comprising: insulating said machines with the expanded styrene polymerof claim
 11. 18. A method of fabricating domestic appliances,comprising: assembling said domestic appliances from parts made from theexpanded styrene polymer of claim
 11. 19. A method of manufacturingpackaging materials, comprising: manufacturing said packaging materialswith the expanded styrene polymer of claim 11.